An action potential is a transient depolarization of the membrane potential of excitable cells. They serve two main functions: to transmit and encode information, and to initiate cellular events such as muscular contraction. An action potential results from a transient change to the properties of the cell membrane, from a state where it is much more permeable to K+ than Na+, to a reversal of these permeability properties. Thus during the action potential an influx of Na+ is responsible for the rapid depolarization and an efflux of K+ causes repolarization. This ionic basis of the action potential can be predicted from Nernst equation and is illustrated in the text. Changes to membrane ionic permeability are due to the opening and closing of voltage-gated ion channels, and the properties of such channels explain additional phenomena such as refractoriness, threshold and cellular excitability. Action potentials conduct with a finite velocity along nerve axons, and the actual velocity depends on a number of factors that include: fibre radius, temperature, functional ion channel number and the presence of a myelin sheath. The physical basis of conduction is explained by the local circuit hypothesis. Synaptic transmission of an action potential is explained in terms of excitatory post-synaptic potential (EPSP) generation at the post-synaptic membrane. The facility by which post-synaptic action potential may be developed is explained in terms of temporal and spatial summation as well as the influence of inhibitory transmitters.